1. Field of the Invention
The present invention relates to resistor networks. More particularly, the present invention relates to programmable, integrated circuit resistance networks.
2. Description of the Prior Art
Active and passive integrated circuits are known. Additionally, programmable active integrated circuits are known. However, manufacturers and designers must rely on off-the-shelf standard values for passive devices, such as resistors.
Typically, resistors have been designed as discrete components--with one fixed-value resistor in each component package. More recently, thin film and thick film integrated circuit technology have been applied to the manufacture of resistors and other such passive devices. Arrays of fixed value resistors are now available in standard dual in-line pin (DIP) packages. Typical of such devices are the Thick Film DIP Resistor Networks Manufactured by TRW Corporation of Redondo Beach, Calif.
Placing fixed value resistors in DIP packages is a substantial improvement in the passive device art. A single DIP package can accommodate several fixed value resistors in less space than that required by the equivalent discrete resistors. Additionally, a DIP package of several resistors can be easily inserted as a unit into and soldered to a printed circuit board during manufacturing processes, in contrast to discrete resistors which require a separate insertion operation for each such device.
While representing an improvement over discrete resistors, the DIP resistor networks with fixed standard values often present design problems. Specifically, the network may contain some resistor values that are usable and others that are not usable in a particular circuit. Thus, part of the network occupies space on a printed circuit board without performing a function. In addition, the standard values provided in existing resistor networks are selected by the manufacturer's criteria, which is typically an omnibus attempt to provide fixed values having wide application. Forced to accept the standard values, the designer is often tempted to compromise a particular circuit design in order to render the standard value network usable for production purposes. One way of achieving some additional flexibility with existing DIP resistor networks has been to connect the fixed resistors in series or in parallel and thus obtain new values. However, this approach uses two resistors to achieve a value that should be available with one resistor. Additionally, printed circuit board patterns must be especially designed to accommodate such jury rigging which complicates the circuit board layout procedure. Further, the range of possible resistance values available by this technique is still relatively narrow and usually does not represent the optimum resistance values for the particular circuit of interest.
It is possible to order custom resistor networks having the specific values desired. This approach requires the use of special integrated circuit masks during the network manufacturing process. As a result, such custom chips are quite expensive and usually cannot be justified for the designer or the small or moderate scale manufacturer. Efforts to date to provide resistor networks devoid of the above disadvantages have not met with success.